1. Field of the Invention
The present invention relates to an apparatus for manufacturing silicon single crystals by the Czochralski (Cz) method and a dopant feeding method thereof and, more particularly, to a silicon single crystal growing furnace supplemented with a low melting point dopant feeding instrument and a low melting point dopant feeding method thereof for manufacturing a heavily doped silicon single crystal with a dopant of low melting point.
2. Background of the Related Art
Cz-grown silicon wafers are deliberately doped with a P or N type dopant during the crystal growing process to obtain a customized specific resistivity suitable for an individual semiconductor device fabrication.
Boron (B) is commonly used as a P-type dopant, and its melting point is about 2,180° C. higher than the melting point (about 1,412° C.) of silicon. When a boron-doped silicon single crystal is grown by the Czochralski (CZ) method, to add the dopant a molten silicon liquid the calculated amount of boron is simply located on the bottom of a quartz crucible together with poly-crystalline silicon during a stacking step in the crystal growth process. Any dopant having a melting point greater than 1412° C., such as boron, is called a high melting point dopant in the field of silicon single crystal growth industry, whereas any dopant having a melting point lower than 1,412° C. is called a low melting point dopant such as, for example, Sb (631° C.), red phosphorous (593° C.), As (817° C.), or the like.
When a low melting point dopant such as Sb, P needs to be heavily doped, the above-mentioned doping method is not practical since before polycrystalline silicon has been fully melted during the melting step, the low melting point dopant is melted and evaporated due to its melting point being lower than 1412° C. Thus, the evaporated low melting point dopant is exhausted out of the silicon crystal growing apparatus together with inert gas (e.g. Ar etc.) which is flowing inside the apparatus in order to remove silicon oxide evaporated from the molten silicon liquid. Hence, it is impossible to produce silicon single crystals having a desired low specific resistivity due to the loss of the dopant. If the apparatus is capsuled to prevent the evaporated low melting point dopant from being exhausted out of the furnace, the generation of oxide particles is enhanced. These particles act as heterogeneous nucleation sites and often prevent effective production of a silicon single crystal. In addition, silicon oxide (SiOx) evaporated from molten silicon liquid remains in the furnace and contaminates the molten silicon and the inside of the apparatus, degrading the quality of crystal.
In the previous art, the low melting point dopant is directly added by dispersing the dopant on the surface of a molten silicon liquid through a feed hopper located a few feet from the molten silicon liquid after poly-crystalline silicon has been completely melted. In this case, the dopant falls to be completely dissolved in the molten silicon liquid because about 30% of the dopant evaporates to be exhausted out of the apparatus together with inert gas. Hence, it is impossible to accurately control dopant concentration in the molten silicon liquid. In addition, the generation of oxide particles is enhanced due to impurities existing in the metallurgical dopant of low degree of purity. As a consequence, a great deal of oxides float on the surface of the molten silicon liquid and act as a source of particles hit, thereby resulting in the failure to grow a silicon single crystal.